The commercial production and application of high temperature superconducting (HTS) wire is well established. High temperature superconductor materials are type II superconductors with a high superconducting transition temperature (Tc), typically such that Tc>77K. Electromagnetic coils wound from HTS wire can achieve high magnetic fields due to the extremely low-levels of heat dissipation at high current densities when operated at temperatures below Tc. In order to maintain the temperature below Tc it is necessary to contain the superconducting coil within a cryostat that is coupled to a cooling source. Electrical current is usually supplied to an HTS superconducting circuit via an electrical circuit which includes current leads made from a normal conducting metal. These current leads penetrate the cryostat wall to link the superconducting circuit to an electrical current source located outside of the cryostat enclosure. Such current leads are a significant source of heat load upon the cryogenic environment due to both heat conduction along, and ohmic dissipation within, the current leads, which are operated at high currents. In addition, the accompanying electrical current source and high current cables have a large footprint, are not easily portable, and are expensive.
Low temperature superconducting (LTS) wire (such as NbTi) is often used to form electromagnetic coils. LTS wire can be joined with superconducting joints, allowing fully superconducting circuits to be manufactured. Fully superconducting circuits can be excited with an external current source in such a way as to maintain a persistent superconducting current around the circuit after the external current source is removed. At present it is not practical to achieve a superconducting joint between HTS conductors in a manufacturing situation. Hence HTS circuits cannot operate in persistent current mode, and it is necessary to leave the external current source connected at all times to balance losses in the resistive joints in the superconducting circuit.
Electromagnetic induction has been used to generate a current within the superconducting circuit without physical connection to the circuit. Apparatus which employ this approach have previously been referred to as a “superconducting DC dynamo” or a “superconducting flux pump”. The term “flux pump” is used to refer to a wide range of devices which induce either persistent bulk magnetisation within a bulk superconducting material or produce a net current to flow around a superconducting circuit. In this specification the term “superconducting current pump” refers to a device which induces a net current to flow around a superconducting circuit. The term “rotating flux pump” and “current pump” are used interchangeably in this specification.
Superconducting flux pumps can be broadly classified as either switched-type flux pumps or rotating-type flux pumps. A switched flux pump has no moving parts and flux pumping is achieved by operating switches in the circuit. For example international patent application publication WO2010/070319 reports a flux pump that magnetises a bulk piece of high temperature superconductor (HTS) material through the use of switchable magnetic material. Rotating flux pumps have moving parts including a rotor which moves relative to a stator containing part of a superconducting circuit to be energized. The rotor carries a source of magnetic flux, such as one or more permanent magnets. The rotor is positioned close to the stator so that the magnetic flux from the source penetrates a section of the superconducting circuit and traverses the superconductor to induce a current in the superconducting circuit. A rotating flux pump requires that the gap between rotor and stator to be less than a few millimeters, to generate sufficient flux density in the superconducting circuit elements at the stator to enable current pumping to occur. The rotating parts are positioned inside the cryostat, and a mechanical coupling penetrates the cryostat wall to connect the rotor to a source of rotational motion, such as an electrical motor. Rotating flux pumps have been used with Type I or Type II superconducting materials which have a variety of transition temperatures ranging from NbTi (Tc=9.2 K) to YBCO (Tc≈95 K). The rotor and stator may be conveniently arranged in either a radial-flux geometry or an axial-flux geometry or a combination thereof. Radial-flux geometry means the rotor and stator pieces are arranged about a common concentric axis such that flux links cross a radial gap between the stator and rotor. Axial-flux geometry means the stator and rotor pieces are displaced linearly along a common axis such that flux links across an axial gap formed between the stator and rotor. For example, the flux pump arrangement reported in international patent application publication WO2012/018265 has a rotor carrying a series of permanent magnets which rotate in close proximity to an HTS stator wire to induce a current in the superconducting circuit.
It is an object of the invention to improve upon known superconducting flux pumps and their application to a superconducting circuit including an electromagnetic coil or at least to provide the public with a useful choice.
In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.